1. Field of the Invention
The present invention relates generally to the fabrication of fuel assembly support grids for nuclear reactors, and more particularly to an improved method of fabricating a fuel assembly grid using a flared sleeve.
2. Description of the Related Art
In most pressurized water nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. These fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids that are spaced axially along the fuel assembly length and are attached to a plurality of elongated thimble tubes of the fuel assembly. The thimble tubes typically receive control rods, plugging devices, or instrumentation therein. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the ends of the thimble tubes that extend slightly above and below the ends of the fuel rods.
The grids, as is known in the relevant art, are used to precisely maintain the spacing between the fuel rods in the reactor core, resist rod vibration, provide lateral support for the fuel rods and, to some extent, vertically restrain the rods against longitudinal movement. The grids are typically made of materials such as stainless steel, Inconel, and alloys of Zirconium. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of cells which individually accept the fuel rods therein. Depending upon the configuration of the thimble tubes, the thimble tubes can either be received in cells that are sized the same as those that receive fuel rods therein, or can be received in relatively larger thimble cells defined in the interleaved straps.
The straps are configured such that the cells each include a plurality of relatively compliant springs and a plurality of relatively rigid dimples, with the springs and dimples being formed into the metal of the interleaved straps and protruding outwardly therefrom. The springs and dimples of each cell engage the respective fuel rod extending through the cell. Outer straps of the grid are attached together and peripherally enclose the inner straps of the grid to impart strength and rigidity to the grid.
Because the straps that make up a grid are relatively thin, i.e., on the order of 0.0105 inches in some cases and 0.008 inches in others, they tend to be flexible, making it difficult to securely attach (such as by welding) the lengthy thimble tubes directly to the straps. Instead, it has become common practice to first attach a relatively shorter cylindrical sleeve made of, for example, stainless steel or alloys of Zirconium, to each of the cells of the straps that are to receive a thimble tube by welding the sleeve and straps together. Next, each of the thimble tubes is inserted through the corresponding sleeves and is attached to the sleeves, and thus the grid, by bulging the thimble tubes out at a location either above or below each grid.
FIG. 1 is a partial isometric view of a prior art sleeve 5 attached to the straps 10 of a grid. In prior art grid fabrication methods, sleeve 5 is placed on top of and aligned with cell 15 formed by straps 10. Cell 15 is provided with a generally circular shape for mating with sleeve 5 by providing a radius on straps 10 at that point. Sleeve 5 is then butt welded to straps 10 at butt weld joint 20 shown in FIG. 1. The butt welding of sleeve 5 to straps 10 has proven to be problematic primarily due to the fact that there is normally a gap between sleeve 5 and straps 10 that form cell 15, a condition known as poor sleeve to strap fit-up. This condition is the result of the difficulty in providing cell 15 with a perfectly circular, appropriately sized shape for mating with sleeve 5. This gap makes it difficult to effectively join sleeve 5 and straps 10.
In addition, as is known, the straps, such as straps 10, used to construct fuel assembly grids are elongated sheets of metal. Each sheet is formed with a plurality of parallel slots that extend transversely down approximately one half of the strap. For hexagonally shaped grids made with three sets of interconnected straps, the first set of aligned straps has all of its slots extending downwardly from the top edge of the strap, the second set of straps has all of its slots extending upwardly from the bottom edge of the strap, and the third set of straps has half of its slots extending downwardly from the top edge of the strap and half of its straps extending upwardly from the bottom edge of the strap in an alternating fashion. As is known, this grid is constructed by inserting the slots of certain of the straps into the slots of certain other of the straps to form an interconnecting lattice pattern. The result is a number of what are known as loose straps. Loose straps are the portions of the straps adjacent to each slot which, because of the slot, remain free to move after the straps are interconnected with one another. In order to prevent any potential strap or spring to rod fretting resulting from the movement of the loose straps, the loose straps are fixed in place by attaching them either to the solid portion of an adjacent strap or to a sleeve in the case of loose straps that are positioned adjacent to a sleeve. In the prior art, loose straps are so attached by laser seam welding a weld tab, commonly referred to as a sail, provided on the straps to the adjacent strap or sleeve, as the case may be. This is shown in FIG. 1 at weld seam 25. Again, problems have arisen due to the fact that gaps between each sleeve and the adjacent loose straps make it difficult to effectively join the two materials.
Another problem that exists with prior art sleeves, such as sleeve 5 shown in FIG. 1, and prior art grid fabrication methods is that, due to poor sleeve to strap fit-up, the straps forming a cell for receiving a thimble tube will often encroach on the path of the thimble tube (the straps do not form a perfect circle such that some portions may extend inside the circumference of the sleeve), thus causing difficulty during thimble tube loading. In addition, in the prior art, thimble tube loading has also been adversely impacted as a result of the weld heat affected zone entering the inside diameter of the straps and sleeve, causing the internal diameter thereof to become smaller after grid welding due to the shrinkage of the material in that area.
Thus, there is a need for a grid fabrication method that alleviates sleeve to grid attachment problems and thimble tube loading problems caused by poor sleeve to strap fit-up.